Caulobacter is a single-celled organism which differentiates during its normal cell cycle. This differentiation provides well-defined landmarks which mark progression through the cell cycle. Consequently, Caulobacter is an excellent model system for studies on the control of cell cycle events. Since its differentiation occurs under constant environmental conditions in the absence of cell-cell interactions, Caulobacter must have some kind of internal clock responsible for the timing of cell cycle events. At present, the investigator's hypothesis is that part of the "clock" comes from the ability of Caulobacter to monitor its progress through checkpoints in the cell cycle by using a series of membrane-bound senior proteins. Each sensor protein would change its conformation in response to the completion of a particular cell cycle event. The conformational change would result in the autophosphorylation of the sensor with the subsequent transfer of the phosphate group to the corresponding response regulator. To test this hypothesis, Dr. Ely proposes to determine when during the cell cycle the pleC gene is expressed; to determine the cellular location of the PleC protein; to determine how the expression of the pleC gene is regulated; to identify genes involved in polar organelle development which are regulated by the PleC sensor protein; to identify the response regulator(s) which work with PleC to regulate genes involved in polar organelle development, and to identify and characterize other phosphoproteins which regulate cell cycle-dependent gene expression in Caulobacter.